1. Field of the Invention
The present invention relates to an optical system for a projector, and particularly to such an optical system for a projector as incorporates a reflection-type spatial light modulator, such as a digital mirror device.TM. (or DMD.TM. for short, manufactured by Texas Instruments Incorporated; hereinafter referred to simply as a digital mirror device or DMD), that is provided with a large number of variable-reflection-angle mirror elements that can vary the reflection angle of the light incident thereon in accordance with a video signal so that only the light corresponding to the video signal will be reflected toward a projection optical system.
2. Description of the Prior Art
In recent years, as higher and higher resolution is desired in images in general, also in the field of projectors, development of techniques has been sought that achieve a substantial increase in the number of pixels without making the optical system larger. One attempt to meet such needs is the development of a projector employing a DMD.
A DMD is produced by forming a large number of minute rectangular high-reflectance mirror elements, of which the inclination is variable in accordance with a video signal, on a silicon memory chip by a process as used to manufacture a semiconductor device. A projector employing such a DMD, by varying the inclination of those mirror elements, controls the direction in which it reflects the illumination light incident thereon in such a way as to converge only the desired parts of the reflected light on a screen and thereby project a desired image thereon.
In accordance with a video signal, the individual mirror elements of a DMD, when in an on state, reflect light toward a projection optical system and, when in an off state, change their inclination to reflect light in a different direction so that the light will not enter the projection optical system. However, owing to restrictions imposed on the design of a projector, the light reflected from the mirror elements when they are in an off state is shone on a side-wall portion of a prism constituting a prism system disposed between the DMD and the projection optical system, and therefore there is a possibility of this light being further reflected therefrom so as to enter the projection optical system.
In particular, when the prism is surrounded by a medium, such as air, having a small refractive index, most of the above-mentioned light shone on the side-wall portion of the prism is reflected from the inner surface of the side-wall portion of the prism, and this greatly increases the amount of such secondary-reflection light. This secondary-reflection light (unnecessary, or stray, light), when it enters the projection optical system, may cause another image (a ghost) separate from the normal image to appear on the screen.
One way to prevent such entry of secondary-reflection light into the projection optical system is to form the side-wall portion of the prism into a diffusive surface that diffuses the light incident thereon. Another way is to paint the outer surface of the side-wall portion of the prism black, or to vapor-deposit on that surface a light-absorbing dielectric film, or to affix to that surface a member having a bottom surface so shaped as to absorb light so that this bottom surface will absorb light and convert it into heat.
The method of absorbing light and converting it into heat by the use of a member having a bottom surface so shaped as to absorb light is adopted, for example, in the projector-oriented optical system disclosed in Japanese Laid-Open Patent Application H9-96867. In this optical system, a heat-dissipating member having a comb-tooth-shaped bottom surface is arranged so as to face a side wall of a prism, with a shock-absorbing pad in between, in such a way that this bottom surface is kept in close contact with the shock-absorbing pad. In this optical system, the recessed portions of the comb-tooth-shaped bottom surface absorb light and convert it into heat, and the heat-dissipating member as a whole dissipates the resulting heat.
However, even if light is diffused, it is inevitable that part of the diffused light will enter the projection optical system. It is practically impossible to form a black thin film that completely absorbs the light incident thereon, and therefore, even if a black thin film is applied, some light, left unabsorbed, may enter the projection optical system. It is difficult to produce a member having so intricate a shape as to absorb completely the light incident thereon, and therefore it is inevitable that part of the incident light will be reflected or diffused in the vicinity of the protruding portions of the comb-tooth-shaped bottom surface of this member.